I have noticed in the past that ..compared to the n8x0's chargers..
the usb chargers for the n900 can be more finicky ..
I have had some n900 chargers that worked fine forever and a day...
others that worked fine or seemed to ....and then after a few years started becoming problematic...
In the last 5 years I have found 2 n900 chargers (official nokia ones...at that...) that I labeled problematic and set aside..

If the battery is fine...
and the internal connection is fine..
and it works fine with another charger...
Then I suspect the charger is the culprit...or a leading cause at the least...
(based on what odd behaviour I have come to see.)